In order to observe the dynamic physiology of the brain, a variety of sensors are required to be arranged around the head of a human (or animal) subject. These sensors are typically electrodes for detecting the electroencephalographic signal, but the may also include optical or magnetic sensors. For accurate analysis of the sources of physiological signals, it is also necessary to obtain precise measurements of the three-dimensional spatial coordinates of the sensors, so that they may be registered with images of the head made using other modalities (such as MRI or PET). Thus, a specialized art has developed for the purpose of measuring the locations of these sensors.
In the previous art, one method uses a fixed transmitter of electromagnetic radiation, and a wand containing an electromagnetic detector which may be manually positioned at the location of each sensor in turn. The position of the wand may be evaluated by analysis of the signals received by the detecting wand. (See for example U.S. Pat. No. 4,812,812.) This method is accurate but time-consuming, and may be susceptible to artifacts caused by metal objects in the vicinity of the subject.
Another method uses an elastic cap stretched over the head, with fiduciary markings that may be used to pre-mark the head of the subject with the target locations of the sensors (U.S. Pat. No. 5,293,867). This method is also time-consuming and impractical, especially with high-density electrode systems such as the Geodesic Sensor Net (our U.S. Pat. No. 5,291,888.)
Furthermore, some very specialized means have been developed for detecting the positions of sensors in magnetoencephalograph (MEG) systems, such as U.S. Pat. No. 4,995,395. However, these means are not useful with electroencephalographic sensors, except when used in conjunction with an MEG system.
The use of stereoscopic imaging to determine the positions of objects is, in general, well known to the existing art of photogrammetry (through triangulation, for example). However, in some important respects, this existing art is inadequate to meet the needs of EEG sensor localization.
In order to measure the positions of all sensors simultaneously at one moment in time (preventing artifacts caused by motion of the subject or by motion of the sensors relative to each other) it is necessary to surround the subject with cameras, to be used to capture images simultaneously. A number of patents have previously recognized the utility of such a multiple camera structure, such as U.S. Pat. No. 5,745,126 (Jain et al) and U.S. Pat. No. 6,084,979 (Kanade et al.) These patents were aiming to develop a “virtual reality” representation of objects in space. However, they do not appear to have realized that a minimal structure necessary to completely surround a regular convex object (such as a human head) is only eleven cameras in an icosahedral gantry. This choice results in a widely spaced set of cameras (with a dihedral angle of approximately sixty degrees), leading to another set of novel problems.
In the existing art, local measures of image correlation are often used to establish the level of disparity or dihedral angle between images. However, at very large angles this becomes problematic, because the varying angles result in varying amounts of local foreshortening of the image. This foreshortening is not necessarily predictable, depending on the orientation of objects in the field of view of the cameras. Therefore, well-known methods for establishing local correlation such as found in U.S. Pat. No. 6,480,620 (Sakamoto); U.S. Pat. No. 5,963,664 (Kumar, et al); U.S. Pat. No. 5,548,326 (Michael) and so forth, are unlikely to be successful in this case. The problem is further exacerbated by the fact that these local correlation algorithms also assume a locally planar surface containing the objects under observation; in our case the shape of the head is strongly curved.
Another well-known methodology uses “structured light” to establish a set of points which may be readily identified across multiple images. These methods are described in patents such as U.S. Pat. No. 6,341,016 (Malione) and U.S. Pat. No. 5,969,722 (Palm). However, we are interested in precisely locating the centers of the sensor objects, and with a structured light system there can be no guarantee that any particular sensor will have its shape fully characterized with adequate precision. Also, it is difficult to imagine the extension of the “structured light” paradigm, into the requirement of multiple simultaneous viewpoints surrounding the head.